1. Field of the Invention
The invention is a spectrometer and, more particularly, a solid-state device to analyze the wavelength in an electromagnetic wave (spectrophotometer). In a preferred embodiment of the invention, the device is produced from quantum-well semiconductor materials.
2. Discussion of the Background
Multi quantum-well radiation detectors (MQWRD), using transitions between both captive and free levels and between different captive levels are known to industry. Their operation will now be briefly described.
They consist of an alternating stack of small-gap semiconductor (SGS) layers between two large-gap semiconductors (LGS) as shown in FIG. 1a. The energy difference between the lower end of the conductive band of the two semiconductors is the band-offset .DELTA. E. For example, these two semi-conductors can be GaAs for the SGS and Al.sub.x Ga.sub.1-x As (where x is between 0 and 1). The electrons in such a structure are exposed to a potential well .DELTA. E deep and d wide, where d is the thickness of the SGS layer. If the width d is sufficiently small, the electron energy corresponding to movement perpendicular to the layers is quantized in levels E.sub.1, E.sub.2, etc.
In captive-free photoconductive detectors, level E.sub.1 is captive (E.sub.1 &lt;.DELTA. E) and level E.sub.2 is free (E.sub.2 .ltoreq..DELTA. E). If an electron is placed at level E.sub.1 (for example by doping), a photon whose energy h .upsilon. exceeds .DELTA. E-E.sub.1 will cause an optical transition (see FIG. 1b). The electron is then free to move and can be detected as a current across the terminals of a multi quantum-well (see FIG. 1c).
Optical modulators, using asymmetric quantum wells (AQW) are also known to industry. FIGS. 2a and 2b represent an example of an asymmetric quantum well. The quantum well consists of two small-gap semiconductors SGS1 and SGS2 with .DELTA. E1&gt;.DELTA. E2. For example, SGS1 can be GaAs and SGS2 can be Al.sub.x Ga.sub.1-x As. This quantum well is confined between two barriers formed by a large-gap semiconductor (LGS) which can be in Al.sub.y Ga.sub.1-y As where y is between 0 and 1 and greater than x.
The stacks are produced such that there are at least two captive levels E'.sub.1 and E.sub.2 ' in the well. A photo whose energy h .upsilon..sub.o =E.sub.2 '-E.sub.1 ' is then absorbed in the well (FIG. 2b). These quantum wells can be stacked to achieve an asymmetric multi quantum well (AMQW) as shown in FIG. 2c.
When an electric field F is applied to terminals CO1 and CO2 of the AMQW, the position of levels E'.sub.1 and E'.sub.2 changes: this is the linear STARK effect. The photon resonant wavelength then becomes: EQU h.upsilon.(F)=h.upsilon..sub.o +qF.delta..sub.12
where q is the electron charge and .delta..sub.12 is the distance between the mean electron positions on level E'.sub.1 and level E'.sub.2 (see FIG. 3a). The absorption peaks then shift towards higher energies (shorter wavelengths) for F&gt;0 (FIG. 3b) and to lower energies for F&lt;0.
As shown on FIG. 3c, the modulator absorption peak can therefore be shifted part of a given wavelength towards shorter wavelengths (F&gt;0) or longer wavelengths (F&gt;0).